The waters of the North Sea, the Baltic Sea, and of Norway’s fjords and coasts, are becoming darker as a result of climate change, according to new research. The darkening is primarily being caused by the increasing levels of organic compounds in the waters, brought to the coasts via regional rivers and lake drainage.
This darker water has been resulting in fewer marine areas with fish, and more with jellyfish. The jellyfish benefit from the darker waters, while the fish have a harder time competing with them because of the increased darkness.
According to University of Bergen marine biologist, Dag L. Aksnes, the process has probably been occurring over many decades. And there is clear evidence that recent changes in weather patterns and in the climate are accelerating it.
“This fresh water contains far more coloured dissolved organic matter (CDOM) than marine water, so our coastal waters are darkening,” explains Professor Aksnes.
The ongoing research has been focused on two specific fjords, Lurefjorden and Masfjorden, located in northern Hordaland county on Norway’s western coast. The fjords were selected because they are similar in many ways and also located near each other, but they possess one important difference: Masfjorden has a far higher salinity and is more sea-like than Lurefjorden, “which contains lower-salinity coastal water all the way down to its seabed.”
The research has found that while Masfjorden is still home to large numbers of fish, its darker neighbor Lurefjorden is now largely populated by the jellyfish Periphylla periphylla.
“Periphylla periphylla is a very light-sensitive jellyfish that thrives best in the world’s very deep marine waters,” continues Professor Aksnes. “But the water in Lurefjorden has now become so murky and dark that it probably is helping this jellyfish to thrive. At the same time, the fjord has become less hospitable as a habitat for important fish species.”
These darker waters make it more difficult for predatory fish to hunt, as most are largely reliant on sight. Sightless jellyfish on the other hand aren’t hindered at all, being “tactile predators that drift quite randomly and rely on collisions to find the food they eat.”
“The problem is not that the fish in Lurefjorden lack food. On the contrary, the numbers of prey organisms are far higher than in many other fjords. But since the predatory fish see so poorly in murky water, they are quite simply having difficulty finding enough food. So the jellyfish have practically no competition for the abundant prey organisms.”
The changing light conditions have also lead to changes in algal photosynthesis and further production of organic compounds. The research has demonstrated that “changes normally associated with eutrophication (nutrient pollution) and human emissions of nutrients can also cause the water to darken. This shrinks the euphotic zone — the layer of water with adequate light penetration for phytoplankton such as algae to carry out photosynthesis — reducing the abundance of attached algae such as seaweed and kelp while boosting the growth of planktonic algae. Meanwhile, nutrient concentration rises and oxygen saturation in the marine layers below the surface declines.”
The increased precipitation predicted for the region because of global climate change is expected to result in similar changes. “More precipitation means that more murky fresh water mixes with the coastal water, making it less saline and murkier,” explains Professor Aksnes.
“Furthermore, studies done at the University of Oslo indicate that increased precipitation and rising temperatures lead to changes in vegetation on land, which in turn increases the concentration of CDOM in the fresh water that mixes with the coastal water. We don’t know yet whether this leads to undesired changes in our coastal ecosystems, but if so, it will be hard to reverse,” says professor Aksnes.
Source: The Research Council of Norway